The present invention relates to paper making machines and, more particularly, to a through-air drying paper making machine having a twin-fabric forming section.
A paper making machine necessarily includes a forming section for forming a paper web. The forming section may have different configurations depending on the characteristics desired of the paper web to be formed. For example, a through-air drying (TAD) paper making machine for making tissue typically has a forming section where the web is formed and usually partially dewatered before the web is transferred to a through-air drying fabric for further water removal in at least one through-air drying unit. In some instances, a TAD machine may have a twin wire forming section which forms the tissue paper web between two forming fabrics. Once the paper web is formed, it may be partially dewatered before it is transferred from the forming fabrics to a TAD fabric, wherein the TAD fabric transports the paper web through the TAD units in order to dry the paper web. TAD paper making machines having a twin wire forming section, therefore, usually have at least three fabrics for forming and drying the paper web. Accompanying each fabric in a TAD paper making machine is an associated investment cost, operational and maintenance requirements, and a number of moving parts. Typically, each additional fabric results in a larger footprint occupied by the paper making machine. Also, the TAD fabric in a typical TAD paper making machine usually retains a significant amount of the water removed from the paper web after the paper web has been transported through the TAD unit(s). Ordinarily, the water retained in the TAD fabric must be removed by relatively sophisticated equipment before returning to receive more of the partially dewatered paper web from the twin wire forming section in order to avoid re-watering of the partially dewatered paper web. The provisions necessary for removing water from the TAD fabric thereby add further investment costs and operational expenses to the paper making process. Thus, there exists a need for a simplified TAD paper making machine for minimizing investment costs which is more efficient in terms of operational, maintenance, size, and space concerns.
One example of a paper making machine having a twin wire forming section is disclosed by U.S. Pat. No. 4,102,737 to Morton which describes a process and apparatus for forming a paper web having improved bulk and absorptive capacity. The apparatus disclosed by the Morton ""737 patent comprises a twin wire forming section where a paper web is formed between a forming wire and a foraminous drying/imprinting fabric about a conventional twin wire forming roll. After the paper web is formed between the forming wire and the drying/imprinting fabric, this xe2x80x9claminate sandwichxe2x80x9d passes over a direction changing roll and then across one or more vacuum boxes to increase the fiber consistency of the paper web. The laminate sandwich is then directed about a conventional rubber covered couch roll for changing direction thereof downwardly to a vacuum pickup shoe mounted against the inside face of the drying/imprinting fabric. The forming wire is then separated from the laminate sandwich such that the paper web remains in contact with the drying/imprinting fabric.
However, the apparatus disclosed by the Morton ""737 patent subjects the formed paper web, sandwiched between the forming wire and the drying/imprinting fabric, to two changes of direction of approximately 90 degrees each. The first change of direction occurs after the paper web has been formed about the forming roll, where the laminate sandwich passes over the direction changing roll to be directed across the vacuum boxes. The second change of direction occurs as the laminate sandwich is passed over the couch roll and directed to the vacuum pickup shoe. As shown in FIG. 1 of the ""737 patent, the paper web sandwiched between the two fabrics is thus subject to a total directional change of approximately 180 degrees after it is formed at the forming roll and before the forming wire is separated therefrom. Due to the different distances traversed by each of the inner and the outer fabrics as they pass over a direction changing component, the fabric immediately adjacent the component, or the inner fabric, will tend to shift in position so as to run ahead of the fabric disposed opposite the inner fabric from the component, or the outer fabric, each time the laminate sandwich is subject to a change of direction. Changing the direction of the laminate sandwich therefore causes one of the fabrics to run ahead of the other and thereby causes deleterious shear forces in the paper web disposed therebetween. Thus, there exists a need for a twin wire forming section for a paper making machine capable of forming and dewatering a paper web while minimizing shear forces in the paper web before it is subjected to subsequent downstream water-removing processes.
Thus, there exists a need for a twin wire formation type paper making machine having a lower investment cost with correspondingly lower operational, maintenance, and space concerns. Such a paper making machine should also be capable of forming a paper web and transporting the paper web to subsequent downstream water-removing processes while minimizing deleterious shear forces in the paper web to thereby obtain a more consistent and damage-free paper web.
The above and other needs are met by the present invention which, in one embodiment, provides a paper making apparatus comprising a twin-fabric forming section, a water-removing section arranged downstream of the forming section, and a web transfer device disposed between the forming section and the water-removing section. The forming section includes a forming roll, a pair of fabrics guided to travel about a portion of the circumference of the forming roll, and a headbox operable to deposit a slurry between the fabrics so as to form a paper web therebetween. Preferably, one of the fabrics in the forming section comprises a through-air drying (TAD) fabric arranged to carry the paper web through the water-removing section, while the other fabric comprises a forming wire. At the web transfer device, the paper web is separated from the forming wire and the paper web then carried on the TAD fabric through the water-removing section. The water-removing section includes at least one through-air dryer for at least partially drying the paper web. Most preferably, the forming section and the web transfer device are arranged such that the forming wire and the TAD fabric, with the paper web sandwiched therebetween, travel along a substantially straight path between the forming roll and the web transfer device, thereby resulting in minimal directional changes of the paper web after formation thereof about the forming roll and prior to separation of the paper web from the forming wire.
According to another advantageous embodiment of the present invention, the paper making apparatus may further include a dewatering section, following the forming section, for partially dewatering the paper web. The dewatering section preferably includes a vacuum dewatering device disposed adjacent the forming wire for dewatering the paper web through the forming wire. The dewatering section may further include a pressure dewatering device disposed adjacent the TAD fabric, generally opposite the vacuum dewatering device, for facilitating dewatering of the paper web through the forming wire. Preferably, the pressure dewatering device comprises at least one of a pressure chamber and an air knife. The web transfer device follows the dewatering section and further includes a suction device disposed adjacent the TAD fabric for retaining the paper web on the TAD fabric.
A paper making apparatus according to embodiments of the present invention may further include a web structuring device, following the web transfer device and disposed adjacent the TAD fabric, for structuring and further dewatering the paper web. The paper web is then transported to the water-removing section, wherein the water-removing section is preferably adapted to dry the paper web with minimal compaction thereof. The water-removing section preferably comprises at least one of an inward flow through-air dryer and outward flow through-air dryer. In one embodiment, the water-removing section may comprise a final drying section having at least one through-air dryer for drying the paper web. In an alternate embodiment, the water-removing section may comprise a pre-drying section having at least one through-air dryer for partially drying the paper web and a final drying section having a Yankee dryer for further drying the paper web.
According to an advantageous embodiment of the paper making apparatus of the present invention, the forming wire and the TAD fabric, with the paper web sandwiched therebetween, travel between the forming roll and the web transfer device with less than about 90 degrees of directional change of the paper web after formation thereof and prior to separation of the paper web from the forming wire. In one particularly advantageous embodiment, the paper web experiences between about 2 degrees and about 5 degrees of directional change between the forming roll and the web transfer device. Further, the path between the forming roll and the web transfer device preferably has a minimal length which, in some embodiments, is no longer than about 1.5 meters.
Another advantageous aspect of the present invention comprises a method of producing a web of paper. A paper web is first formed between a forming wire and a through-air drying (TAD) fabric guided to travel about a portion of the circumference of a forming roll in a twin-fabric forming section. The paper web is then separated from the forming wire at a web transfer device such that the paper web is supported by the TAD fabric. The forming section and the web transfer device are preferably arranged such that the forming wire and the TAD fabric, with the paper web sandwiched therebetween, travel along a substantially straight path between the forming roll and the web transfer device. Most preferably, the paper web experiences minimal directional change after formation thereof about the forming roll and prior to separation of the paper web from the forming wire. Following separation of the forming wire, the paper web is then transported on the TAD fabric through a water-removing section having at least one through-air dryer for drying the paper web.
In one embodiment, the method may further include the step of dewatering the paper web through the forming wire following the forming step. Preferably, the dewatering step further comprises dewatering the paper web through the forming wire with at least one of a vacuum dewatering device disposed adjacent the forming wire and a pressure dewatering device disposed adjacent the TAD fabric. In addition, the separating step may further comprise retaining the paper web on the TAD fabric using a suction device disposed adjacent the TAD fabric. Following the separation step, the method may further include the step of structuring the paper web using a web structuring device. The separating step comprises transporting the paper web between the forming roll and the web transfer device with less than about 90 degrees of directional change of the paper web after formation thereof about the forming roll and prior to separation of the paper web from the forming wire. Preferably, the paper web is transported between the forming roll and the web transfer device with between about 2 degrees and about 5 degrees of directional change. The path between the forming roll and the web transfer device preferably has a minimal length which, in some embodiments, is no longer than about 1.5 meters.
Preferably, the transporting step further comprises transporting the paper web through the water-removing section with minimal compaction thereof. In addition, the transporting step preferably comprises transporting the paper web through at least one of an inward flow through-air dryer and an outward flow through-air dryer. In one embodiment, the transporting step further comprises transporting the paper web through a final drying section having at least one through-air dryer for drying the paper web. In an alternate embodiment, the transporting step further comprises transporting the paper web through a pre-drying section having at least one through-air dryer for partially drying the paper web and a final drying section having a Yankee dryer for further drying the paper web.
Thus, it will be appreciated that embodiments of the present invention utilize two fabrics for forming the paper web and transporting the paper web through a water-removing section. Utilizing two fabrics, instead of the three fabrics typically required in a TAD paper making machine having a twin wire forming section, provides a TAD paper making machine requiring a lower investment and which minimizes operational, maintenance, and space concerns. The elimination of one of the three fabrics typically required in a TAD paper making machine thereby reduces the investment cost and the operational and maintenance concerns usually associated with each additional fabric. For example, it becomes unnecessary to remove the water retained by the TAD fabric since the TAD fabric is configured to receive the wet paper web directly thereon in the forming section. Since less components are required according to embodiments of the present invention than with a three fabric system, a more compact paper making machine is obtained which generally requires less space for installation due to a smaller footprint. Further, the arrangement of the components of the paper making apparatus according to embodiments of the present invention, such that the paper web is transported from about the forming roll to the web transfer device over a relatively short distance with minimal directional change thereof, minimizes deleterious shear forces in the paper web and thereby produces a more consistent and damage-free paper web. A further advantage is realized in the process due to the paper web being formed directly on a relatively coarse and open TAD fabric. The resulting paper web is better formed with a more bulky and structured fiber network, wherein the wet and loose paper fiber typically has a more readily reorganizable structure for filling the openings in the TAD fabric to form a more desirable fiber network. It will be recognized, therefore, that the present invention facilitates the achievement of these and other distinct advantages over prior art paper making devices.